Group III nitride semiconductors that are represented by gallium nitride have been attracting attention as materials for light-emitting diodes (LEDs) and laser diodes (LDs) because blue-violet light emission with a high efficiency is obtained therewith. Among others, LDs are expected as light sources fit for large-capacity optical disk apparatus and recent years have seen aggressive development of high-output LDs as light sources for writing thereof.
A typical structure for a nitride-based blue-violet LD is illustrated in FIG. 1. In this structure, a ridge 101 is formed by dry etching. The upper part of the ridge is covered with an insulating film 102 having a stripe-shaped opening and a p type electrode 103 is provided in the opening. Current confinement is attained by using the stripe-shaped electrode, and the control of the transverse mode is achieved by adjusting ridge width and ridge height thereof.
However, such a ridge type semiconductor laser has had problems as described below. In applications to optical disks, in order to efficiently focus laser beams in spot form, it is necessary to adjust the pattern of the beams, and thus it is necessary to control the transverse mode thereof so that a far-field pattern of laser beams has a Gaussian type intensity profile. For this purpose, in a high-output blue LD, it is necessary to narrow the ridge width to such a narrow size as 1.7 μm. However, as the ridge width is narrowed, the area of the electrode provided thereon is narrowed, resulting in the increase of the contact resistance. In a high-output LD, because of a high operating current density, there are some cases where the deterioration of the device may be induced by a heat generated at the contact.
In view of such a problem, inner stripe type LDs in which AlN and (Al)GaN are employed to form the current-confining layer as shown in FIG. 2 have been proposed (JP 2001-15860 A1, JP 10-093192 A1, and JP 2003-78215 A1). Because a wide contact area can be employed in these inner stripe type LDs, a low contact resistance can be realized even in a high-output LD with a narrow stripe width. In particular, the inner stripe structure in which low-temperature grown AlN is used to form the current-confining layer (JP 2001-15860 A1) has the advantage that it is less influenced by damage during the formation of the opening of the current-confining layer and contamination with impurities, and hence it is laid a great hope to as a high-output LD capable of operating with a low-voltage.
On the other hand, there have been also examined such a trial that a superlattice structure is used to form a cladding layer in the upper part of an active layer (JP 2002-171028 A1). Because a carrier is induced to the interface of each layer composing the superlattice structure, the resistance in the thickness-direction of the layers decreases, and the carrier mobility in the in-plane direction of the layers increases. As a result of this, it becomes possible to efficiently utilize the carrier in the operation of the element and to substantially reduce the operating voltage.